Journal of Colloid and Interface Science 435 (2014) 21–25
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Mechanism of enhanced nitrate reduction via micro-electrolysis at the powdered zero-valent iron/activated carbon interface Jinghuan Luo a, Guangyu Song a, Jianyong Liu a,⇑, Guangren Qian a, Zhi Ping Xu b,⇑ a b
School of Environmental and Chemical Engineering, Shanghai University, 333 Nanchen Road, Shanghai 200444, PR China ARC Centre of Excellence for Functional Nanomaterials, Australian Institute for Bioengineering and Nanotechnology, The University of Queensland, Brisbane, QLD 4072, Australia
a r t i c l e
i n f o
Article history: Received 21 June 2014 Accepted 22 August 2014 Available online 30 August 2014 Keywords: Nitrate reduction Fe0/AC composite Micro-electrolysis system Electron transfer
a b s t r a c t Nitrate reduction by zero-valent iron (Fe0) powder always works well only at controlled pH lower than 4 due to the formation of iron (hydr)oxides on its surface. Fe0 powder combined with activated carbon (AC), i.e., Fe0/AC micro-electrolysis system, was first introduced to enhance nitrate reduction in aqueous solution. Comparative study was carried out to investigate nitrate reduction by Fe0/AC system and Fe0 under near-neutral conditions, showing that the Fe0/AC system successfully reduced nitrate even at initial pH 6 with the reduction efficiency of up to 73%, whereas for Fe0 only 10%. The effect of Fe0 to AC mass ratio on nitrate reduction efficiency was examined. Easier nitrate reduction was achieved with more contact between Fe0 and AC as the result of decreasing Fe0 to AC mass ratio. Ferrous ion and oxidation–reduction potential were measured to understand the mechanism of enhanced nitrate reduction by Fe0/AC microelectrolysis. The results suggest that a relative potential difference drives much more electrons from Fe0 to AC, thus generating adsorbed atomic hydrogen which makes it possible for nitrate to be reduced at near-neural pH. Fe0/AC micro-electrolysis thus presents a great potential for practical application in nitrate wastewater treatment without excessive pH adjustment. Ó 2014 Published by Elsevier Inc.
1. Introduction Nitrate contamination in ground and surface water mainly originates from agricultural runoff, animal wastes, septic systems, industrial processes, etc. and has become an increasingly severe environmental issue [1]. Nitrate will pose eutrophication of water when discharged excessively into aquatic systems, or serious threat to human health when reduced to nitrite, causing methemoglobinemia, cancer, liver damage, and so forth [2,3]. Contributed to high cost effectiveness and easy operation [4], nitrate reduction using zero-valent iron (Fe0) powder has therefore been extensively investigated in recent decades. Unfortunately, considerable studies [4–6] have revealed that the performance of Fe0 in nitrate reduction is strongly dependent on the pH of aqueous solution, and the rapid process only occurs at pH < 4. The acidic condition must be maintained by adding acid solution in order to enhance iron corrosion and dissolve ferrous (hydr)oxides on the Fe0 surface to keep nitrate being reduced at an appropriate rate ⇑ Corresponding authors. Fax: +86 21 66137761 (J. Liu). Fax: +61 7 33463973 (Z.P. Xu). E-mail addresses: [email protected] (J. Liu), [email protected] (Z.P. Xu). http://dx.doi.org/10.1016/j.jcis.2014.08.043 0021-9797/Ó 2014 Published by Elsevier Inc.
[5,6]. Fortunately, nanoscale zero-valent iron (NZVI) has been demonstrated to be a promising alternative even under near-neutral and/or neutral conditions due to its large specific surface area and high reactivity [7–9]. Nonetheless, this process also faces several critical issues, such as easy aggregation [10], difficult separation/recycling [7], or tedious preparation when supporting NZVI onto other carriers [7,11], thus hindering its field application. Other processes, including ferrous ion augmenting [12–14] and copper deposition [15,16], have been also proposed to promote nitrate reduction by iron under near-neutral or neutral conditions. Fe2+ augmenting and Fe/Cu bimetallic system facilitate direct or indirect electron transfer from iron to nitrate [13,16] and both have achieved encouraging results. However, they also have their own drawbacks in nitrate reduction, since the former is possibly required to remove excessive ferrous ion to avoid taste and/or odor problems [13], and the latter is short-lived owing to loss of loosely bound copper particles as well as formation of a passive oxide layer [17]. Therefore, it is necessary to find an alternative more preferable and practical to maintain nitrate reduction using Fe0 within a wider range of solution pH. Currently, zero-valent iron/activated carbon (Fe0/AC) microelectrolysis has been widely applied in treatment of various
22
J. Luo et al. / Journal of Colloid and Interface Science 435 (2014) 21–25
wastewaters, such as landfill leachate [18], coking [19] and dye [20] wastewater, for its easy operation, low cost and high efficiency [21]. In an Fe0/AC system, when iron (anode) and activated carbon (cathode) particles are mixed and contacted with each other, massive microscopic galvanic cells will be formed spontaneously between these two electrodes [18,22]. AC could thus enhance reduction efficiency by transferring electrons from galvanic corrosion of iron to the contaminant [23]. Under anaerobic conditions, organic pollutants are also significantly reduced by atomic hydrogen and Fe(II) released from the micro-electrolysis process [24]. Therefore, this research was particularly designed to examine whether the Fe0/AC micro-electrolysis formed through incorporating AC into Fe0 enhances nitrate reduction even in near-neutral aqueous solutions in comparison with Fe0 only, and whether the micro-electrolysis can be applied as a much more convenient and practical technology for nitrate reduction without using massive acid. Moreover, the reaction process will be characterized to elucidate what the possible mechanism is for nitrate reduction enhancement by micro-electrolysis. Hence, this work was carried out to (i) investigate enhanced nitrate reduction by Fe0/AC system under acidic to near-neutral conditions as compared with Fe0 only, (ii) understand the effect of Fe0 to AC mass ratio on nitrate reduction efficiency during microelectrolysis process, and (iii) illustrate the possible mechanism for enhanced nitrate reduction by Fe0/AC system after monitoring variations in ferrous ion and oxidation–reduction potential (ORP).
(UV-4802, UNICO, USA). ORP and pH were measured using a PHS3C ORP-meter and a Mettler-toledo pH-meter, respectively. 3. Results and discussion 3.1. Comparison of nitrate reduction by Fe0/AC, Fe0, Fe0/sand and AC Fig. 1 shows nitrate reduction as a function of reaction time by Fe0/AC, Fe0, Fe0/sand and AC at initial pH 3. It can be found that nitrate was substantially reduced by 89% after 120 min using Fe0/ AC micro-electrolysis, much higher than that (9%) using Fe0 only. This enhanced efficiency for nitrate reduction by Fe0/AC system is mainly attributed to the introduction of AC particles, which leads to the formation of numerous Fe0/AC microscopic galvanic cells [21]. The cell, constructed with Fe0 as the anode and AC as the cathode, will naturally be developed once two kinds of particles are contacted with each other in nitrate solution, thus accelerating iron corrosion and promoting nitrate reduction. In comparison, only 22% of nitrate reduction efficiency using Fe0/sand system indicates just a little bit effect of iron-granules dispersion (to avoid massive cementation) by inert particles on the reduction enhancement. Interestingly, as also shown in Fig. 1, Fe0/AC and Fe0 system performed similar nitrate reduction (19% vs. 9%) within the first 40 min, but strikingly, the nitrate concentration using Fe0/AC system was then sharply decreased while the other one almost remained unchanged.
2. Materials and methods
3.2. Effect of initial pH
2.1. Materials
To understand the effect of initial solution pH on nitrate reduction by Fe0/AC and Fe0, batch tests at initial pH 2–6 were carried out within 720 min (Fig. 2a). At pH 2, Fe0/AC and Fe0 system showed approximately the same nitrate reduction efficiency, i.e., 97% and 93%, respectively. However, with increasing initial pH, two systems performed discriminately in nitrate reduction. The efficiency by Fe0/AC decreased yet with all above 70%, whereas that by Fe0 only dropped drastically to around 10%. This much poorer reduction efficiency by Fe0 at initial pH higher than 2 has also been observed in other works, since the process is acid-promoted and the solution pH can severely affect iron corrosion, thus determining nitrate reduction efficiency [5]. Remarkably, the reduction efficiency by Fe0/AC system remained 73% even at initial pH 6, mainly attributed to the role of numerous Fe0/AC galvanic cells formed during micro-electrolysis process rather than iron corrosion only.
2.2. Experimental procedures Batch tests for nitrate reduction were conducted in 250 ml of headspace bottles which were introduced with 200 ml of nitrate solution, and desired initial pH was adjusted by dilute HCl or NaOH solution. Before adding iron and/or activated carbon, the nitrate solution in headspace bottle was purged with high-purity nitrogen gas for 15 min to eliminate dissolved oxygen. The sealed bottles were then shaken using a rotary shaker at ambient temperature, and aliquots of samples (2.0 ml) were withdrawn by syringe at the designated time to measure the concentrations of NO 3 –N, 3+ NH4+–N, Fe2+, and ORP, pH (NO undetectable). Diluted 2 –N or Fe by 25 times, the withdrawn sample was divided in duplicate, one alkalified by NaOH solution to precipitate ferrous ion and filtered through a 0.22 lm membrane filter for NO3–N/NH4+–N analysis, and the other only filtered for Fe2+ analysis. 2.3. Analytical methods
1.0
0.8
NO3--N c/c0
Commercial iron powder was purchased from Sinopharm Chemical Reagent Co., Ltd, China. Prior to each experiment, iron powder was pre-treated with 0.1 M HCl for 10 min to remove oxides covering on its surface, and then washed 15 times with distilled water before being dried in a vacuum drying oven (105 °C). Activated carbon and silica sand powders were washed 10 times and also dried under 105 °C. All of above particles were sieved through a 100 mesh sieve. Sodium nitrate of analytical grade and deionized water were employed to prepare nitrate aqueous solution.
0.6
0.4
Fe 0/AC Fe0 Fe0/sand AC
0.2
0.0 0
According to the Standard Methods for the Examination of Water and Wastewater (American Public Health Association), NO3–N, NH4+–N with Nesster’s reagent and Fe2+ with phenanthroline were determined colorimetrically using a UV–vis spectrophotometer
20
40
60
80
100
120
Time (min) Fig. 1. Nitrate reduction by Fe0/AC, Fe0, Fe0/sand and AC (initial conditions: pH = 3, Fe0 dosage = 40 g/l, Fe0 to AC mass ratio = 3:1, Fe0 to sand mass ratio = 2:1, and nitrate concentration = 50 mg N/l).
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J. Luo et al. / Journal of Colloid and Interface Science 435 (2014) 21–25 100
Fe0/AC Fe0
1.0
pH=2 pH=6
0.8
NO3--N c/c0
NO3--N reduction (%)
80
60
40
20
0.6
0.4
a
b
0.2
0.0
0 2
3
4
5
6
0
200
pH
400
600
800
Time (min)
Fig. 2. Effect of initial pH on nitrate reduction: (a) efficiencies by Fe0/AC and Fe0 after 720 min; and (b) kinetics by Fe0/AC (initial conditions: Fe0 dosage = 40 g/l, Fe0 to AC mass ratio = 3:1, and nitrate concentration = 50 mg N/l).
Therefore, this Fe0/AC system would be a cost-effective alternative for nitrate reduction under near-neutral conditions without excessive pH adjustment. Fig. 2b further gives kinetics of nitrate reduction by Fe0/AC system at initial pH 2 and 6 (The final pH was 7.2 and 10.0, respectively.). After reaction for 120 min, the nitrate reduction percentage at initial pH 2 was 89%, but only 4% in the case at pH 6. Surprisingly, the reduction process was continued with 68% of nitrate reduced after 720 min at initial pH 6. The much more rapid reduction of nitrate under lower pH condition can be mainly due to a larger amount of protons that not only participate in the corrosion reaction but also dissolve iron (hydr)oxides coating on the Fe0 surface to avoid their inhibition effect, with the similar mechanism to nitrate reduction by zero-valent iron [5]. Nonetheless, the nitrate reduction rate at higher pH was much slower, indicating an obviously different reaction mechanism. It is our belief that electron transfer could be the control step in nitrate reduction by Fe0/AC micro-electrolysis at initial pH 6.
2:1, there would be more AC particles and easier contact with Fe0 granules when assuming their particle size to be similar. Therefore, much more micro-electrolysis reactions are promoted, with much easier galvanic corrosion and consequently nitrate reduction [25]. Correspondingly, the ammonium nitrogen concentration, as shown in Fig. 3b, clearly increased as nitrate reduction continued. Mainly resulting from ammonium adsorption by iron [26] and/or activated carbon [27], the NH4+–N generated was somewhat less than NO3–N reduced. More interestingly, however, the increase of NH4+–N concentration was approximately the same as the decrease of NO3–N concentration between two different Fe0 to AC mass ratio, which suggests ammonium as the major reduction product. Ammonium, if necessary, can be removed by stripping under a strong alkaline condition.
3.4. Mechanism study 3.4.1. Ferrous ion Fig. 4 presents the variations of ferrous ion and pH during nitrate reduction by Fe0/AC and Fe0. Obviously, the ferrous ion was almost totally precipitated from the initial concentrations of 46 mg/l and 22 mg/l at 20 min during these two processes, respectively, as the solution pH both increased monotonically from initial 3 to above 9 [24]. Note that the ferrous concentration by Fe0/AC was 14–24 mg/l higher than Fe0 only before reaction for 80 min,
3.3. Effect of Fe0 to AC mass ratio As illustrated in Fig. 3a, the final reduction efficiency of nitrate increased from 37% to 87% with the Fe0 to AC mass ratio decreased from 5:1 to 2:1. This efficiency increase could be explained by the growing quantity of galvanic cells when the Fe0 to AC mass ratio decreased. With the decreasing Fe0 to AC mass ratio from 5:1 to
2:1
3:1
4:1
1.0
80
NH4+-N (mg/l)
0.8
NO3--N c/c0
5:1
0.6
0.4
60
40
20 0.2
a
b
0
0.0 0
20
40
60
80
Time (min)
100
120
0
20
40
60
80
100
120
Time (min)
Fig. 3. Effect of Fe0 to AC mass ratio on (a) nitrate reduction and (b) ammonium production (initial conditions: pH = 3, Fe0 dosage = 40 g/l, and nitrate concentration = 100 mg N/l).
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J. Luo et al. / Journal of Colloid and Interface Science 435 (2014) 21–25 50 10 40
2+
0
Fe -Fe /AC Fe2+-Fe0 pH-Fe0/AC pH-Fe0
20
6
pH
Fe2+ (mg/l)
8 30
4
10
0
2 0
20
40
60
80
100
3.4.2. Oxidation–reduction potential In order to get better understanding on the enhancement of nitrate reduction by Fe0/AC micro-electrolysis, comparative ORP variation of aqueous solutions in the presence of Fe0/AC and Fe0 was also displayed in Fig. 5. For Fe0 only, ORP was initially 208 mV, but dropped to 207 mV after reaction for 120 min. While for Fe0/AC system, the ORP value decreased much more quickly and was always lower than that for Fe0 with the largest difference of nearly 200 mV, further suggesting a favorable condition for nitrate reduction as a result of galvanic effect. In the circumstance herein, the ORP value for the two systems decreased due to oxidative NO3 consumption, reductive NH+4 and Fe2+ generation as well as pH increase [28], and finally became closer at later stage because of Fe2+ precipitation.
120
Time (min) Fig. 4. Fe2+ and pH variations during nitrate reduction by Fe0/AC and Fe0 (initial conditions: pH = 3, Fe0 dosage = 40 g/l, Fe0 to AC mass ratio = 3:1, and nitrate concentration = 50 mg N/l).
3.4.3. Mechanism of nitrate reduction by micro-electrolysis The mechanism of nitrate reduction by Fe0/AC micro-electrolysis is illustrated as a conceptual model in Fig. 6. In the case using Fe0 only, nitrate is reduced to ammonium by the electron transferred directly from iron corrosion [5,29]:
NO3 þ 4Fe0 þ 7Hþ ! NHþ4 þ 4Fe2þ þ 3OH
ð1Þ
0
While mixing Fe with AC, there will be a relative potential difference within this galvanic cell, which enhances electron transfer from iron to activated carbon. The adsorbed H+ ion or H2O molecule on the AC surface thus accepts the electron and is converted to adsorbed atomic hydrogen (denoted as Had) under acidic or near-neutral condition, respectively. Had, a more powerful reducing species than direct electron transfer [26,30], then rapidly reduces neighboring adsorbed nitrate into ammonium. The proposed reactions can be represented as follows [15,16]. Anodic (oxidative)
200
Fe0/AC Fe0
ORP (mV)
100
0
-100
-200
Fe0 ! 2e þ Fe2þ -300
ð2Þ
Cathodic (reductive) 0
20
40
60
80
100
120
Time (min) Fig. 5. ORP variation during nitrate reduction by Fe0/AC and Fe0 (initial conditions: pH = 3, Fe0 dosage = 40 g/l, Fe0 to AC mass ratio = 3:1, and nitrate concentration = 50 mg N/l).
Hþad
þ e ! Had
ð3Þ
H2 O þ e ! Had þ OH NO3ad
þ 7Had !
NHþ4ad
þ 3OH
ð4Þ
ð5Þ
4. Conclusions indicating an easier iron corrosion drove by a potential difference during micro-electrolysis system (discussed below). This result is significantly vital to enhanced nitrate reduction.
Powdered Fe0/AC composites were investigated for effective nitrate reduction in aqueous solution at pH from 2 to 6. Comparative study showed that Fe0/AC system reduced nitrate at near-neural pH with a reduction efficiency of up to 73%, whereas for Fe0 around 10%, which was also dependent on the Fe0 to AC mass ratio. The enhancement of nitrate reduction is largely
Fig. 6. Conceptual processes during nitrate reduction by (a) Fe0/AC and (b) Fe0.
J. Luo et al. / Journal of Colloid and Interface Science 435 (2014) 21–25
attributed to Fe0/AC micro-electrolysis, through which much more electrons are driven to the AC surface, as proposed in many reports for organics reduction and/or removal from wastewaters [18,19]. Fe0/AC micro-electrolysis has thus shown a great potential application for nitrate reduction owing to its high efficiency and costeffectiveness in a wider pH range, and should further be examined in practice. Acknowledgment This work was financially supported by the Program for Innovative Research Team in University (No. IRT13078). References [1] J.F. Li, Y.M. Li, Q.L. Meng, J. Hazard. Mater. 174 (2010) 188–193. [2] J.H. Zhang, Z.W. Hao, Z. Zhang, Y.P. Yang, X.H. Xu, Process Saf. Environ. Prot. 88 (2010) 439–445. [3] D.W. Cho, C.M. Chon, B.H. Jeon, Y. Kim, M.A. Khan, H. Song, Chemosphere 81 (2010) 611–616. [4] J.M. Rodriguez-Maroto, F. Garcia-Herruzo, A. Garcia-Rubio, C. Gomez-Lahoz, C. Vereda-Alonso, Chemosphere 74 (2009) 804–809. [5] Y.H. Huang, T.C. Zhang, Water Res. 38 (2004) 2631–2642. [6] C.P. Huang, H.W. Wang, P.C. Chiu, Water Res. 32 (1998) 2257–2264. [7] Y. Zhang, Y.M. Li, J.F. Li, L.J. Hu, X.M. Zheng, Chem. Eng. J. 171 (2011) 526–531. [8] Y.H. Hwang, D.G. Kim, H.S. Shin, J. Hazard. Mater. 185 (2011) 1513–1521. [9] F.S. Fateminia, C. Falamaki, Process Saf. Environ. Prot. 91 (2013) 304–310.
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[10] Z.M. Jiang, S.J. Zhang, B.C. Pan, W.F. Wang, X.S. Wang, L. Lv, W.M. Zhang, Q.X. Zhang, J. Hazard. Mater. 233 (2012) 1–6. [11] Z.M. Jiang, L. Lv, W.M. Zhang, Q.O. Du, B.C. Pan, L. Yang, Q.X. Zhang, Water Res. 45 (2011) 2191–2198. [12] Y.H. Huang, T.C. Zhang, Water Res. 39 (2005) 1751–1760. [13] T. Suzuki, M. Moribe, Y. Oyama, M. Niinae, Chem. Eng. J. 183 (2012) 271–277. [14] J. Xu, Z.W. Hao, C.S. Xie, X.S. Lv, Y.P. Yang, X.H. Xu, Desalination 284 (2012) 9– 13. [15] Y.H. Liou, S.L. Lo, C.J. Lin, C.Y. Hu, W.H. Kuan, S.C. Weng, Environ. Sci. Technol. 39 (2005) 9643–9648. [16] Y.H. Liou, C.J. Lin, S.C. Weng, H.H. Ou, S.L. Lo, Environ. Sci. Technol. 43 (2009) 2482–2488. [17] C.J. Lin, S.L. Lo, Y.H. Liou, J. Hazard. Mater. 116 (2004) 219–228. [18] D.W. Ying, J. Peng, X.Y. Xu, K. Li, Y.L. Wang, J.P. Jia, J. Hazard. Mater. 229 (2012) 426–433. [19] W.W. Liu, X.Y. Tu, X.P. Wang, F.Q. Wang, W. Li, Chem. Eng. J. 200 (2012) 720– 728. [20] L. Qin, G.L. Zhang, Q. Meng, L.S. Xu, B.S. Lv, Chem. Eng. J. 210 (2012) 575–584. [21] H.M. Zhou, P. Lv, Y.Y. Shen, J.J. Wang, J. Fan, Water Res. 47 (2013) 3514–3522. [22] B. Lai, Y.X. Zhou, P. Yang, J.H. Yang, J.L. Wang, Chemosphere 90 (2013) 1470– 1477. [23] F.P. Van der Zee, I.A.E. Bisschops, G. Lettinga, J.A. Field, Environ. Sci. Technol. 37 (2003) 402–408. [24] X.Y. Yang, J. Hazard. Mater. 169 (2009) 480–485. [25] X.M. Dou, R. Li, B. Zhao, W.Y. Liang, J. Hazard. Mater. 182 (2010) 108–114. [26] L. Gui, R.W. Gillham, M.S. Odziemkowski, Environ. Sci. Technol. 34 (2000) 3489–3494. [27] R. Boopathy, S. Karthikeyan, A.B. Mandal, G. Sekaran, Environ. Sci. Pollut. Res. 20 (2013) 533–542. [28] Z.Q. Shi, J.T. Nurmi, P.G. Tratnyek, Environ. Sci. Technol. 45 (2011) 1586–1592. [29] M.J. Alowitz, M.M. Scherer, Environ. Sci. Technol. 36 (2002) 299–306. [30] J. Wang, J. Farrell, Environ. Sci. Technol. 37 (2003) 3891–3896.
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Mechanism of enhanced nitrate reduction via micro-electrolysis at the powdered zero-valent iron/activated carbon interface Jinghuan Luo a, Guangyu Song a, Jianyong Liu a,⇑, Guangren Qian a, Zhi Ping Xu b,⇑ a b
School of Environmental and Chemical Engineering, Shanghai University, 333 Nanchen Road, Shanghai 200444, PR China ARC Centre of Excellence for Functional Nanomaterials, Australian Institute for Bioengineering and Nanotechnology, The University of Queensland, Brisbane, QLD 4072, Australia
a r t i c l e
i n f o
Article history: Received 21 June 2014 Accepted 22 August 2014 Available online 30 August 2014 Keywords: Nitrate reduction Fe0/AC composite Micro-electrolysis system Electron transfer
a b s t r a c t Nitrate reduction by zero-valent iron (Fe0) powder always works well only at controlled pH lower than 4 due to the formation of iron (hydr)oxides on its surface. Fe0 powder combined with activated carbon (AC), i.e., Fe0/AC micro-electrolysis system, was first introduced to enhance nitrate reduction in aqueous solution. Comparative study was carried out to investigate nitrate reduction by Fe0/AC system and Fe0 under near-neutral conditions, showing that the Fe0/AC system successfully reduced nitrate even at initial pH 6 with the reduction efficiency of up to 73%, whereas for Fe0 only 10%. The effect of Fe0 to AC mass ratio on nitrate reduction efficiency was examined. Easier nitrate reduction was achieved with more contact between Fe0 and AC as the result of decreasing Fe0 to AC mass ratio. Ferrous ion and oxidation–reduction potential were measured to understand the mechanism of enhanced nitrate reduction by Fe0/AC microelectrolysis. The results suggest that a relative potential difference drives much more electrons from Fe0 to AC, thus generating adsorbed atomic hydrogen which makes it possible for nitrate to be reduced at near-neural pH. Fe0/AC micro-electrolysis thus presents a great potential for practical application in nitrate wastewater treatment without excessive pH adjustment. Ó 2014 Published by Elsevier Inc.
1. Introduction Nitrate contamination in ground and surface water mainly originates from agricultural runoff, animal wastes, septic systems, industrial processes, etc. and has become an increasingly severe environmental issue [1]. Nitrate will pose eutrophication of water when discharged excessively into aquatic systems, or serious threat to human health when reduced to nitrite, causing methemoglobinemia, cancer, liver damage, and so forth [2,3]. Contributed to high cost effectiveness and easy operation [4], nitrate reduction using zero-valent iron (Fe0) powder has therefore been extensively investigated in recent decades. Unfortunately, considerable studies [4–6] have revealed that the performance of Fe0 in nitrate reduction is strongly dependent on the pH of aqueous solution, and the rapid process only occurs at pH < 4. The acidic condition must be maintained by adding acid solution in order to enhance iron corrosion and dissolve ferrous (hydr)oxides on the Fe0 surface to keep nitrate being reduced at an appropriate rate ⇑ Corresponding authors. Fax: +86 21 66137761 (J. Liu). Fax: +61 7 33463973 (Z.P. Xu). E-mail addresses: [email protected] (J. Liu), [email protected] (Z.P. Xu). http://dx.doi.org/10.1016/j.jcis.2014.08.043 0021-9797/Ó 2014 Published by Elsevier Inc.
[5,6]. Fortunately, nanoscale zero-valent iron (NZVI) has been demonstrated to be a promising alternative even under near-neutral and/or neutral conditions due to its large specific surface area and high reactivity [7–9]. Nonetheless, this process also faces several critical issues, such as easy aggregation [10], difficult separation/recycling [7], or tedious preparation when supporting NZVI onto other carriers [7,11], thus hindering its field application. Other processes, including ferrous ion augmenting [12–14] and copper deposition [15,16], have been also proposed to promote nitrate reduction by iron under near-neutral or neutral conditions. Fe2+ augmenting and Fe/Cu bimetallic system facilitate direct or indirect electron transfer from iron to nitrate [13,16] and both have achieved encouraging results. However, they also have their own drawbacks in nitrate reduction, since the former is possibly required to remove excessive ferrous ion to avoid taste and/or odor problems [13], and the latter is short-lived owing to loss of loosely bound copper particles as well as formation of a passive oxide layer [17]. Therefore, it is necessary to find an alternative more preferable and practical to maintain nitrate reduction using Fe0 within a wider range of solution pH. Currently, zero-valent iron/activated carbon (Fe0/AC) microelectrolysis has been widely applied in treatment of various
22
J. Luo et al. / Journal of Colloid and Interface Science 435 (2014) 21–25
wastewaters, such as landfill leachate [18], coking [19] and dye [20] wastewater, for its easy operation, low cost and high efficiency [21]. In an Fe0/AC system, when iron (anode) and activated carbon (cathode) particles are mixed and contacted with each other, massive microscopic galvanic cells will be formed spontaneously between these two electrodes [18,22]. AC could thus enhance reduction efficiency by transferring electrons from galvanic corrosion of iron to the contaminant [23]. Under anaerobic conditions, organic pollutants are also significantly reduced by atomic hydrogen and Fe(II) released from the micro-electrolysis process [24]. Therefore, this research was particularly designed to examine whether the Fe0/AC micro-electrolysis formed through incorporating AC into Fe0 enhances nitrate reduction even in near-neutral aqueous solutions in comparison with Fe0 only, and whether the micro-electrolysis can be applied as a much more convenient and practical technology for nitrate reduction without using massive acid. Moreover, the reaction process will be characterized to elucidate what the possible mechanism is for nitrate reduction enhancement by micro-electrolysis. Hence, this work was carried out to (i) investigate enhanced nitrate reduction by Fe0/AC system under acidic to near-neutral conditions as compared with Fe0 only, (ii) understand the effect of Fe0 to AC mass ratio on nitrate reduction efficiency during microelectrolysis process, and (iii) illustrate the possible mechanism for enhanced nitrate reduction by Fe0/AC system after monitoring variations in ferrous ion and oxidation–reduction potential (ORP).
(UV-4802, UNICO, USA). ORP and pH were measured using a PHS3C ORP-meter and a Mettler-toledo pH-meter, respectively. 3. Results and discussion 3.1. Comparison of nitrate reduction by Fe0/AC, Fe0, Fe0/sand and AC Fig. 1 shows nitrate reduction as a function of reaction time by Fe0/AC, Fe0, Fe0/sand and AC at initial pH 3. It can be found that nitrate was substantially reduced by 89% after 120 min using Fe0/ AC micro-electrolysis, much higher than that (9%) using Fe0 only. This enhanced efficiency for nitrate reduction by Fe0/AC system is mainly attributed to the introduction of AC particles, which leads to the formation of numerous Fe0/AC microscopic galvanic cells [21]. The cell, constructed with Fe0 as the anode and AC as the cathode, will naturally be developed once two kinds of particles are contacted with each other in nitrate solution, thus accelerating iron corrosion and promoting nitrate reduction. In comparison, only 22% of nitrate reduction efficiency using Fe0/sand system indicates just a little bit effect of iron-granules dispersion (to avoid massive cementation) by inert particles on the reduction enhancement. Interestingly, as also shown in Fig. 1, Fe0/AC and Fe0 system performed similar nitrate reduction (19% vs. 9%) within the first 40 min, but strikingly, the nitrate concentration using Fe0/AC system was then sharply decreased while the other one almost remained unchanged.
2. Materials and methods
3.2. Effect of initial pH
2.1. Materials
To understand the effect of initial solution pH on nitrate reduction by Fe0/AC and Fe0, batch tests at initial pH 2–6 were carried out within 720 min (Fig. 2a). At pH 2, Fe0/AC and Fe0 system showed approximately the same nitrate reduction efficiency, i.e., 97% and 93%, respectively. However, with increasing initial pH, two systems performed discriminately in nitrate reduction. The efficiency by Fe0/AC decreased yet with all above 70%, whereas that by Fe0 only dropped drastically to around 10%. This much poorer reduction efficiency by Fe0 at initial pH higher than 2 has also been observed in other works, since the process is acid-promoted and the solution pH can severely affect iron corrosion, thus determining nitrate reduction efficiency [5]. Remarkably, the reduction efficiency by Fe0/AC system remained 73% even at initial pH 6, mainly attributed to the role of numerous Fe0/AC galvanic cells formed during micro-electrolysis process rather than iron corrosion only.
2.2. Experimental procedures Batch tests for nitrate reduction were conducted in 250 ml of headspace bottles which were introduced with 200 ml of nitrate solution, and desired initial pH was adjusted by dilute HCl or NaOH solution. Before adding iron and/or activated carbon, the nitrate solution in headspace bottle was purged with high-purity nitrogen gas for 15 min to eliminate dissolved oxygen. The sealed bottles were then shaken using a rotary shaker at ambient temperature, and aliquots of samples (2.0 ml) were withdrawn by syringe at the designated time to measure the concentrations of NO 3 –N, 3+ NH4+–N, Fe2+, and ORP, pH (NO undetectable). Diluted 2 –N or Fe by 25 times, the withdrawn sample was divided in duplicate, one alkalified by NaOH solution to precipitate ferrous ion and filtered through a 0.22 lm membrane filter for NO3–N/NH4+–N analysis, and the other only filtered for Fe2+ analysis. 2.3. Analytical methods
1.0
0.8
NO3--N c/c0
Commercial iron powder was purchased from Sinopharm Chemical Reagent Co., Ltd, China. Prior to each experiment, iron powder was pre-treated with 0.1 M HCl for 10 min to remove oxides covering on its surface, and then washed 15 times with distilled water before being dried in a vacuum drying oven (105 °C). Activated carbon and silica sand powders were washed 10 times and also dried under 105 °C. All of above particles were sieved through a 100 mesh sieve. Sodium nitrate of analytical grade and deionized water were employed to prepare nitrate aqueous solution.
0.6
0.4
Fe 0/AC Fe0 Fe0/sand AC
0.2
0.0 0
According to the Standard Methods for the Examination of Water and Wastewater (American Public Health Association), NO3–N, NH4+–N with Nesster’s reagent and Fe2+ with phenanthroline were determined colorimetrically using a UV–vis spectrophotometer
20
40
60
80
100
120
Time (min) Fig. 1. Nitrate reduction by Fe0/AC, Fe0, Fe0/sand and AC (initial conditions: pH = 3, Fe0 dosage = 40 g/l, Fe0 to AC mass ratio = 3:1, Fe0 to sand mass ratio = 2:1, and nitrate concentration = 50 mg N/l).
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J. Luo et al. / Journal of Colloid and Interface Science 435 (2014) 21–25 100
Fe0/AC Fe0
1.0
pH=2 pH=6
0.8
NO3--N c/c0
NO3--N reduction (%)
80
60
40
20
0.6
0.4
a
b
0.2
0.0
0 2
3
4
5
6
0
200
pH
400
600
800
Time (min)
Fig. 2. Effect of initial pH on nitrate reduction: (a) efficiencies by Fe0/AC and Fe0 after 720 min; and (b) kinetics by Fe0/AC (initial conditions: Fe0 dosage = 40 g/l, Fe0 to AC mass ratio = 3:1, and nitrate concentration = 50 mg N/l).
Therefore, this Fe0/AC system would be a cost-effective alternative for nitrate reduction under near-neutral conditions without excessive pH adjustment. Fig. 2b further gives kinetics of nitrate reduction by Fe0/AC system at initial pH 2 and 6 (The final pH was 7.2 and 10.0, respectively.). After reaction for 120 min, the nitrate reduction percentage at initial pH 2 was 89%, but only 4% in the case at pH 6. Surprisingly, the reduction process was continued with 68% of nitrate reduced after 720 min at initial pH 6. The much more rapid reduction of nitrate under lower pH condition can be mainly due to a larger amount of protons that not only participate in the corrosion reaction but also dissolve iron (hydr)oxides coating on the Fe0 surface to avoid their inhibition effect, with the similar mechanism to nitrate reduction by zero-valent iron [5]. Nonetheless, the nitrate reduction rate at higher pH was much slower, indicating an obviously different reaction mechanism. It is our belief that electron transfer could be the control step in nitrate reduction by Fe0/AC micro-electrolysis at initial pH 6.
2:1, there would be more AC particles and easier contact with Fe0 granules when assuming their particle size to be similar. Therefore, much more micro-electrolysis reactions are promoted, with much easier galvanic corrosion and consequently nitrate reduction [25]. Correspondingly, the ammonium nitrogen concentration, as shown in Fig. 3b, clearly increased as nitrate reduction continued. Mainly resulting from ammonium adsorption by iron [26] and/or activated carbon [27], the NH4+–N generated was somewhat less than NO3–N reduced. More interestingly, however, the increase of NH4+–N concentration was approximately the same as the decrease of NO3–N concentration between two different Fe0 to AC mass ratio, which suggests ammonium as the major reduction product. Ammonium, if necessary, can be removed by stripping under a strong alkaline condition.
3.4. Mechanism study 3.4.1. Ferrous ion Fig. 4 presents the variations of ferrous ion and pH during nitrate reduction by Fe0/AC and Fe0. Obviously, the ferrous ion was almost totally precipitated from the initial concentrations of 46 mg/l and 22 mg/l at 20 min during these two processes, respectively, as the solution pH both increased monotonically from initial 3 to above 9 [24]. Note that the ferrous concentration by Fe0/AC was 14–24 mg/l higher than Fe0 only before reaction for 80 min,
3.3. Effect of Fe0 to AC mass ratio As illustrated in Fig. 3a, the final reduction efficiency of nitrate increased from 37% to 87% with the Fe0 to AC mass ratio decreased from 5:1 to 2:1. This efficiency increase could be explained by the growing quantity of galvanic cells when the Fe0 to AC mass ratio decreased. With the decreasing Fe0 to AC mass ratio from 5:1 to
2:1
3:1
4:1
1.0
80
NH4+-N (mg/l)
0.8
NO3--N c/c0
5:1
0.6
0.4
60
40
20 0.2
a
b
0
0.0 0
20
40
60
80
Time (min)
100
120
0
20
40
60
80
100
120
Time (min)
Fig. 3. Effect of Fe0 to AC mass ratio on (a) nitrate reduction and (b) ammonium production (initial conditions: pH = 3, Fe0 dosage = 40 g/l, and nitrate concentration = 100 mg N/l).
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J. Luo et al. / Journal of Colloid and Interface Science 435 (2014) 21–25 50 10 40
2+
0
Fe -Fe /AC Fe2+-Fe0 pH-Fe0/AC pH-Fe0
20
6
pH
Fe2+ (mg/l)
8 30
4
10
0
2 0
20
40
60
80
100
3.4.2. Oxidation–reduction potential In order to get better understanding on the enhancement of nitrate reduction by Fe0/AC micro-electrolysis, comparative ORP variation of aqueous solutions in the presence of Fe0/AC and Fe0 was also displayed in Fig. 5. For Fe0 only, ORP was initially 208 mV, but dropped to 207 mV after reaction for 120 min. While for Fe0/AC system, the ORP value decreased much more quickly and was always lower than that for Fe0 with the largest difference of nearly 200 mV, further suggesting a favorable condition for nitrate reduction as a result of galvanic effect. In the circumstance herein, the ORP value for the two systems decreased due to oxidative NO3 consumption, reductive NH+4 and Fe2+ generation as well as pH increase [28], and finally became closer at later stage because of Fe2+ precipitation.
120
Time (min) Fig. 4. Fe2+ and pH variations during nitrate reduction by Fe0/AC and Fe0 (initial conditions: pH = 3, Fe0 dosage = 40 g/l, Fe0 to AC mass ratio = 3:1, and nitrate concentration = 50 mg N/l).
3.4.3. Mechanism of nitrate reduction by micro-electrolysis The mechanism of nitrate reduction by Fe0/AC micro-electrolysis is illustrated as a conceptual model in Fig. 6. In the case using Fe0 only, nitrate is reduced to ammonium by the electron transferred directly from iron corrosion [5,29]:
NO3 þ 4Fe0 þ 7Hþ ! NHþ4 þ 4Fe2þ þ 3OH
ð1Þ
0
While mixing Fe with AC, there will be a relative potential difference within this galvanic cell, which enhances electron transfer from iron to activated carbon. The adsorbed H+ ion or H2O molecule on the AC surface thus accepts the electron and is converted to adsorbed atomic hydrogen (denoted as Had) under acidic or near-neutral condition, respectively. Had, a more powerful reducing species than direct electron transfer [26,30], then rapidly reduces neighboring adsorbed nitrate into ammonium. The proposed reactions can be represented as follows [15,16]. Anodic (oxidative)
200
Fe0/AC Fe0
ORP (mV)
100
0
-100
-200
Fe0 ! 2e þ Fe2þ -300
ð2Þ
Cathodic (reductive) 0
20
40
60
80
100
120
Time (min) Fig. 5. ORP variation during nitrate reduction by Fe0/AC and Fe0 (initial conditions: pH = 3, Fe0 dosage = 40 g/l, Fe0 to AC mass ratio = 3:1, and nitrate concentration = 50 mg N/l).
Hþad
þ e ! Had
ð3Þ
H2 O þ e ! Had þ OH NO3ad
þ 7Had !
NHþ4ad
þ 3OH
ð4Þ
ð5Þ
4. Conclusions indicating an easier iron corrosion drove by a potential difference during micro-electrolysis system (discussed below). This result is significantly vital to enhanced nitrate reduction.
Powdered Fe0/AC composites were investigated for effective nitrate reduction in aqueous solution at pH from 2 to 6. Comparative study showed that Fe0/AC system reduced nitrate at near-neural pH with a reduction efficiency of up to 73%, whereas for Fe0 around 10%, which was also dependent on the Fe0 to AC mass ratio. The enhancement of nitrate reduction is largely
Fig. 6. Conceptual processes during nitrate reduction by (a) Fe0/AC and (b) Fe0.
J. Luo et al. / Journal of Colloid and Interface Science 435 (2014) 21–25
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